1. Field of the Invention
The present invention relates to an optical fiber ribbon stranding device for superimposing a plurality of optical fiber ribbons and accommodating the optical fiber ribbons thus superimposed into a corresponding one of spiral grooves provided in the outer circumferential surface of a spacer or slotted rod. The present invention also relates to a device for practicing that method.
2. Description of Related Art
An optical fiber cable is provided with aso-called spacer or slotted rod to protect optical fibers. The spacer is formed so that a plastic material, such as polyethylene or the like, is extrusion-molded on and around a tensile body, such as a steel wire, a steel stranded wire, FRP or the like, to have a plurality of spiral grooves in its outer circumferential surface.
FIG. 3 is a perspective view showing an example of such spacer. The spacer 20 includes: a tensile body 21 consisting of a steel wire, a steel stranded wire, FRP or the like; an accommodation body 22 consisting of a plastic material such as polyethylene or the like; and grooves 23 formed spirally in the outer circumferential surface of the accommodation body 22. The spiral direction of the grooves may be reversed to that shown in FIG. 3. The spacer 20 can accommodate, in these grooves 23, various optical fibers including an individual optical fiber in which a glass fiber is coated with a primary coating (consisting of one or more resin layers), and a united optical fibers (i.e. an optical fiber ribbon) in which a plurality of such individual optical fibers are arranged in parallel and further coated together with a secondary coating.
FIG. 4(A) is a cross-sectional view showing an example of such optical fiber cable, and FIG. 4(B) is a cross-sectional view of an optical fiber ribbon. A plurality of individual optical fibers 33a each of which is formed of a glass fiber coated with ultraviolet curing resin or the like as a primary coating are arranged in parallel, and entirely coated with coating 33b of ultraviolet curing resin or the like as a secondary coating, thereby forming an optical fiber ribbon 33.
An accommodation body 32 consisting of a plastic material and having a plurality of spiral grooves 32a is formed on and around a tensile body 31 consisting of a steel wire, a steel stranded wire, FRP or the like, thereby forming a spacer. A plurality of optical fiber ribbons 33 are superimposed one on another and accommodated in each of the grooves 32a of the spacer. A wrapping 34 is applied onto the spacer. An external coating 35 consisting of a plastic material or the like is provided on the wrapping 34. Then, an optical fiber cable is formed.
To manufacture the optical fiber cable of this type, a stranding step of superimposing and accommodating the optical fiber ribbons in each of spiral grooves of a spacer is required. FIG. 5(A) is a front view showing an example of a stranding device used for practicing such stranding step. A spacer 41 is fed out of a reel 38, advanced to the right straight by a take-up device (a belt capstan) 45 while being braked by a tension giving device (a belt capstan) 39, and then taken up on a reel 46. At least the tension giving device 39 and the take-up device 45 are rotated around their axes in association with the advance of the spacer 41 in the direction of the spiral grooves of the spacer 41. That is, the devices 39 and 45 are rotated at the same cycle as the spiral grooves are rotated in association with the advance of the spacer 41. Consequently, each of the grooves extending between the tension giving device 39 and the take-up device 45 is viewed as if it is kept in the same position with respect to a fixed point of the earth.
As illustrated, optical fiber ribbons 42 are fed out of a plurality of reels 37 respectively, passed through a groove formed in a guide die 40, and introduced into a groove of the spacer 41 advanced in the above-described manner. In this case, since the groove of the spacer 41 intersects the guide die 40 in the same position with respect to the earth, the guide die 40 can introduce the optical fiber ribbons 42 into the groove of the spacer 41 even if the guide die 40 is arranged stationary. Note that optical fiber ribbons, which are omitted in FIGS. 5(A) and 5(B) to simplify the illustration, are similarly fed out of respective reels and accommodated by the guide die 40 into corresponding grooves of the spacer 41. The reference numeral 43 represents a pressing die; and 44, a spacer in which optical fiber ribbons have been accommodated in the grooves. A rough winding, a tape winding and so on may be applied onto the spacer 44 before or after the spacer 44 is passed through the pressing die 43 depending on the need.
FIG. 5(B) is an enlarged front view of FIG. 5(A), showing how the guide die 40 guides the optical fiber ribbons 42 into the groove of the spacer 41. FIG. 5(c) is a perspective view showing the guide die 40. A plurality of optical fiber ribbons 42 are passed through each of the grooves 40a formed in the guide die 40 to be introduced into a corresponding one of the grooves of the spacer 41.
The guide die 40 is provided with a circular hole 40b circular in section and extending along its central axis so that spacer 41 is passes through the circular hole 40b. Further, a plurality of inwardly projecting protrusions (not shown) are fixedly provided on the inner circumference of the circular hole 40b of the guide die 40 and inserted into the respective grooves of the spacer 41 so that the grooves of the spacer 41 surely intersect the same positions of the guide die 40, respectively, with respect to the fixed point of the earth when the spacer 41 is passed through the guide die 40.
In place of the guide die 40, a thin disc-like plate having a circular central hole and a plurality of circular holes arranged about the central hole may be used to guide a plurality of optical fiber ribbons 42 to the spacer 41.
Such a method is also available that optical fiber ribbons are introduced into grooves of a spacer without using a guide die. FIGS. 6(A) and (B) are a perspective view and a front view, respectively, showing an example of such method, in which a plurality of optical fiber ribbons 42 are introduced one by one into the groove 41a of a spacer 41 while being separated one from another by a spring-like ring (i.e. a spiral separator) 47 so that the optical fiber ribbons 42 are placed in the groove 41a in a superimposed fashion.
Of the manufacturing methods described above, the method employing a guide die to introduce optical fiber ribbons into each groove of a spacer has the following defect: The optical fiber ribbons that are to be superimposed in one groove of the spacer are inserted into corresponding one groove of the guide die, but the mutual positional relationship of the individual optical fiber ribbons is not fixed so that the superimposed state is unstable within the guide die. Consequently, the optical fiber ribbons are likely to be inclined within the groove of the guide die, and the superimposing order of the optical fiber ribbons is changed at the worst. That it, the alignment is apt to be disordered.
In the method employing a spiral separator, the mutual positional relationship of individual optical fiber ribbons is not fixed, so that the optical fiber ribbons are passed through the corresponding ring parts of the spiral separator in a state where the superimposed state is not determined. Since the optical fiber ribbons enter into the corresponding ring parts of the spiral separator in the unstable state, the optical fiber ribbons may be stripped or damaged by the ring parts of the spiral separator.